Airborne opto-mechanical scanners have been found to be very useful devices because they produce a strip map optical image of the terrain overflown by an aircraft. They are capable of covering a very large area in a single flight and provide imagery superficially like aerial photography. By far, the greatest application of line scanners has been in the infrared spectral regions especially in the two so-called atmospheric window bands, namely 3-5 micrometers and 8-14 micrometers in wavelength. To a lesser extent such devices have been used at shorter wavelengths such as the visible spectral band and with active laser scanning at various laser wavelengths. The present invention applies primarily to passive infrared imaging in these two infrared window bands but the invention could apply as well to visible or near infrared wavelengths.
The earliest airborne infrared line scanners used a scanner type called an "axe-head" scanner because they typically employed a scan mirror having two scanning mirror surfaces resembling a chisel or a blunt axe head in form. Numerous scanners of this type are well-known to the art and have flown successfully from the 1960s to the present. Several disadvantages of this type of scanner led to their replacement as the preferred airborne infrared line scan device. Among these disadvantages were the fact that the 45 degree angle placement of each scan mirror surface caused the image to rotate as the scan progressed from one horizon to the other. In fact the amount of rotation is 180.degree. and is symmetrical about the midscan, or nadir position directly below the aircraft. This rotation of the image causes the image of a road, for example, to appear s-shaped even though the road is perfectly straight and aligned parallel to the scan dimension which is transverse to the line of flight of the aircraft. Such s-shaped distortion not only makes the imagery more difficult to interpret but it also limits the number of detectors which can be placed in linear array in the along track, or flight, direction. It is very useful to employ a linear array of detectors aligned parallel to the flight direction because such an array with a plurality of detectors allows more than one channel of information to be collected for each scan. Another way of expressing this advantage is to say that a linear array with a plurality of detectors has a greater along track coverage. As a consequence, for a given velocity-to-height (V/H) ratio it is possible to keep a contiguous and continuous coverage using fewer scans per second. It is a distinct advantage to be able to keep up with the maximum V/H ratio while using a lower scan mirror rotation rate because reliability increases, electrical power to the scan drive is greatly reduced, and windage and noise are also reduced. Early scanners were not used for high V/H missions because image rotation limited the number of parallel channels to one or two and to provide the increased V/H capability it was necessary to scan at unacceptably high rotation rates such as 12,000 rpm.
Split aperture scanners, such as the Kennedy scanners, began to replace axehead scanners in the 1970 decade. The split aperture type of design had the decided advantage of not causing image rotation as the scan progressed but they used a more complicated optical design and required two optical fold mirrors on either side of the prismatic section scan mirror in order to receive the scanned radiation and fold it ultimately to a focus on the detector focal plane. The split aperture design line scanners used prism shaped scan mirrors which had either three or four facets. Each facet theoretically scans at least 180.degree. of transverse scan angle, but often less than this amount was used because of the inherent distortions in such line scanners. The most annoying of these distortions is called bow-tie distortion and occurs because the fixed along track angular subtense of the linear array of detectors is scanned along the ground transversely and hence, the ground subtense, or ground footprint of the array grows as the scan angle from nadir increases. Because the aircraft moves forward a constant distance during each scan period there will be an eventual overlap beyond a certain scan angle before or after nadir (defined herein as 0 degrees scan angle). This overlap at the higher scan angles causes a loss of information that renders the imagery useless at scan angles in excess of +/-60.degree. unless channels are dropped from the image recording process in a dynamic manner as the scan progresses from nadir. Prior to reaching nadir it is necessary to start with one or two channels (depending on V/H) and to stepwise symmetrically add channels up to the maximum number required at nadir for a given V/H ratio. This technique was known in the prior art but because of electronic complexity, was seldom used until the decade of the 1980s when digital electronic signal processing was added to reconnaissance systems.
The split aperture design provides a reasonably uniform aperture over the major portion of the required fields of view specified by the user community up until the mid 1980s. This feature results from the fact that as one aperture decreases during the scan the other aperture increases. At nadir both apertures are equal. The split aperture designs sum both images at the focal plane and one disadvantage of such designs is the requirement to adjust and maintain this alignment over the full range of vibration and thermal environments of the intended use. Another disadvantage of the split aperture designs is that the across track dimension of each single aperture varies with the scan angle and, hence, its diffraction modulation transfer function (MTF) can have a very low value at certain scan angles, thus degrading the resolution from that aperture. It is not possible to consider the dimension of the combined across track aperture in the evaluation of the diffraction MTF because the infrared radiation is not coherent by virtue of the fact that the optical path length to the focal plane differs between the two adjacent left and right apertures by many wavelengths. It is common practice in the design of such split aperture optics to employ a much larger dimension for the along track (ALT) aperture as compared to the across track (ACT) aperture. For example, at nadir, where the across track (ACT) apertures are equal one could make each ACT aperture to be one inch wide and employ an ALT aperture of 5 inches in length. This causes a drastic difference in the MTF values from diffraction in the two dimensions. This disadvantage is usually overcome by the use of electronic MTF boost in each of the signal processing amplifiers to increase the system across track MTF to acceptable levels and hence, to achieve the desired resolution. An undesirable consequence of the MTF boost is that electronic noise is also boosted and thus, the overall signal-to-noise ratio is diminished. Both the axehead scanners and the split aperture scanners have a limited collecting aperture in proportion to their overall size. The axehead scanner has a lower scan efficiency in comparison to the split aperture types because it typically uses only one or two scan mirror facets per revolution and if more facets are used the scan mirror axehead becomes impractically large. It should be noted that the axehead scanner has a constant aperture MTF with scan angle and if only one 45 degree angle scan mirror facet is used to fill the telescope aperture it is possible to have a large aperture and hence, a large collection area in a small overall cylindrical envelope. This fact has been known to those skilled in the art for many years and such a variation of the axehead scanner has formed the basis for many radiometric imagers used by NASA and by the scientific and weather community of investigators on satellites and aircraft.
In discussing previous state of the art line scanners it should be noted that there is a continuing and urgent need to make improvements in the following areas.
The present scanners are too large for many applications in pods, smaller aircraft, and especially in remote piloted vehicles (RPVs) where the scanner width in particular is too large for the allowable installation diameter.
The present scanners are too heavy for many of their intended uses. The payload capability of many RPVs is severely limited and even if the RPV is capable of lifting the scanner weight its flight range would be improved with any weight reduction.
The present scanners have an excessively high cost for many uses. It would be very beneficial if a low cost small infrared linescan sensor could be developed for RPV use in applications where there is a high probability that the RPV will be lost or have at best a limited lifetime. It is contemplated that such RPVs would be deployed in large numbers where RPVs offer many cost and human benefits compared to manned airborne reconnaissance.
The present scanners are too difficult to maintain in the field and require expensive aerospace ground equipment and extensive training with skilled personnel.
Present linescan systems generate a very high instantaneous data rate which greatly complicates the recording of the imagery unless a substantial amount of data compression and processing is employed on data received from the line scanner.
The use of axehead scanners is limited to low V/H values. Further, they suffer from image rotation s-distortion. While split aperture scanners are not limited to low V/H values and do not rotate the image, they suffer from bow-tie or overlap distortions. Both types of scanners employ fixed infinity focus optical telescopes. Therefore, when using such scanners, a significant amount of defocus occurs which limits resolution at the closer slant ranges that are encountered at low altitudes up to approximately 1000 ft.